You are overengineering.

Water needs depend first on how much rainfall occurs.

A large healthy plant needs 5 gallons of water per day during the middle of the summer.

The simplest way to solve your problem is to put a timer on your soaker hose and adjust it to 15 minutes per day for the first 3 weeks the plants are in the ground, 30 minutes per day for weeks 4 to 6, 60 minutes per day for weeks 7 and up.

Some soaker hoses weep more water than others.

Water sensors are available that detect the amount of moisture in the subsoil and can activate a timer that turns on the water. If you are into gadgets, get a water sensor, an electric water valve and a relay for 110v ac. Hook it up so the water sensor activates the relay which turns on the water valve. With a bit of tweaking and careful positioning of the sensor, you can automate your water supply.

DarJones

I aint no engineer , but the tools I use is a finger and visual perception..depends on the type of soil...sand and loamy soil don't retain water very well, unless heavily mulched (don't we all do that)...dark clay does get soaked, and can choke a plant because it has little viscosity when damp..trick is when it looks dry on top, check 2 inches down.
I grow in dark clay, some may say mud, but it works for me.
I usually let the plants tell me when they need water, and when they do I use seeper hoses, those flat canvas hoses, and let them run fer about 30 min..and judge the run off.

I just keep the soil constantly moist not letting more than the top 1.5" dry out. I use a seltzer bottle with the bottom cot of and a tiny hole punched in the bottom to water.

I use a $10 - $12 water/ph/light meter that you can get at any hardware/garden store...you just stick it down to root depth and it will tell you how wet the soil is...my goal is to keep the soil at a consistent moist level using 1/4" drip irrigation tubing...

Go to the url I've copied here...they have everything for drip irrigation, including how to calculate your watering needs...

Here is a link that might be useful: Dripworks

Totally agree with your final sentence. Watch the weather and look at your plants. I even look at the weather forecast because I can almost predict that if I water before a few rainy/overcast days I will get cracking fruit and possibly onset of diseases.

Much also depends on your nitrogen fertility. Higher N will always be metabolized by the plants creating a much larger plant size than normal. With a lot of lush new growth there is more plant transpiration going on on hot days. You then need to keep the plant from being stressed because of the monster that you created.

I grew ramapos on a drip system last year (zone 9). It gets 110 degrees in the summer. I used 1/4 gallon per hour drip emmiters for soil that is slightly sandy to loamy. I put the water on an automatic timer set for 2 times a week, 4 hours each watering cycle. My first tomatoes had blossom end rot because i did not water enough. After that I set my timer to 3 times per week at 4 hours each cycle. That was too much water as the tomatoes had vertical cracks where they connect to the skin. I pretty much played with the water cycle the rest of the summer and finally got it right. It seemed that the right cycle depended on the outside air temp. That changed daily. Good luck.

Other considerations:

The soaker hose will leak differently as time passes. Over the course of just one season, the hose will degrade more quickly in some places (like places that get jiggled or that bend) and that can lead to more leakage there, while other parts seem to clog themselves up, so you end up, over time, with a less even water distribution. That's not bad -- just something to consider when making calculations. One simple calculation won't get you through a whole season.

Also, depending on how you have the hose buried, it can end up spraying the underside of your plants with water from holes that develop. If you see that happening, cover it up at that spot with some compost or leaves.

Hey fusion power:

I think your recommendation comes close to the data. You are doubling every 3 weeks which is similar to adding 1x every week.

The key will be to determine the first "x". I'm going to use a meter suggested by someone here.

Do you really think that the peak useage of a single plant is 5 gal per day ? That seems extremely high and I don't think that much water can be supplied each day without going nuts.

Just a thought....my plants often go downhill later in the season, also. Sometimes it is size/water related but sometimes it is fertilizer or/and disease related. I now try to fertilize and mulch well before vacations as well as making watering arrangements. The mulch helps cut down on foliage disease especially when there is heavy rain resulting in soil splashes onto the lower leaves.