ASAP. There are two problems with spring seeding. The main one is that you are germinating your seed at the same time the crabgrass seed is germinating. Crabgrass is a summer annual plant that is highly adapted to fast sprouting and surviving summer heat. The second problem is the new grass seed is not adapted to the summer heat, at least not yet. Had it sprouted in the fall then by the following summer the roots would be better adapted to the heat.

Armed with that information, you can see what to expect. If you get the crabgrass started and you have a warm spring/summer, then you can expect a full crabgrass lawn by July.

If you can afford sod, then this problem will not be nearly as bad.

You can mitigate the crabgrass issue by using Tenacity at seed down. Scotts also makes a starter fert which contains mesotrione (the Tenacity ai) that can be used instead of Tenacity. Dchall makes a good point that the grass will require a lot of water to keep alive during heat, so it would be wise to apply fungicide at seed down and every 2 weeks thereafter until you're out of the woods.

Once seed is established and grass is emerging can I then apply a pre-emergent??

I think that would be too late.

Once seed is established and grass is emerging can I then apply a pre-emergent??

Most pre-emergents have a waiting period for application to newly established lawns. Pre-emergents are root pruners, and with new grass having shallow roots it can get damaged. Tenacity will take you through the first ~21 days as a pre-emergent, then you can start using it as a post-emergent to control anything that comes up until it's safe to apply a regular pre-emergent (about 60 days).

So in the meantime I have used this product before to spot spray would this be safe to use now I am not starting a new lawn just seeding in spots. https://www.amazon.com/Chickweed-Clover-Oxalis-Killer-128/dp/B000HCRM4S/ref=pd_ys_c_rfy_rp_0?_encoding=UTF8&pd_rd_i=B000HCRM4S&pd_rd_r=0EGZRJDPNSJF6JTF92E1&pd_rd_w=4zFX8&pd_rd_wg=uMd80&psc=1&refRID=S31PSVMEM1P7CBZBMZSK

Just for knowledge sake when we talk about germination once the grass begins to sprout and grow has germination then stopped?? So any herbicide or pre-emergent not have an effect on the grass growing??

Just for knowledge sake when we talk about germination once the grass begins to sprout and grow has germination then stopped??

All seed does not germinate at the same time, some will sprout after 4 days, some will take 2 weeks. That's just how nature works. The germination times put forth by the seed producer represent how long it will take for the germination percentage stated on the bag. So, if they say 3 weeks for Kentucky Bluegrass and the bag says says 85% germination, 85% of the seed will germinate within 3 weeks. That doesn't mean that you won't see some grass start to pop up after one week, but it also doesn't mean that just because you see grass pop up after one week it's done germinating.

So any herbicide or pre-emergent not have an effect on the grass growing??

I'm not saying that at all. Most herbicides have clearly stated waiting times for newly seeded lawns, because new grass is far more sensitive to them than established grass. 3-way broadleaf herbicides for example typically require that you wait until the second mowing. Longer-lived pre-emergents like prodiamine require you wait at least 60 days from germination because they impact root development, while some shorter-lived pre-emergents like mesotrione can be applied at seeding time.

Tenacity (mesotrione) is fairly unique in that it can be applied at seeding time as a pre-emergent AND it's fairly safe as a post-emergent on newly seeded lawn. That's not typically the case with most other herbicides. On the flipside, it doesn't cover as broad a range of weeds as other more specialized herbicides, nor does it last as long in the soil. So, it's a great tool to use in the short term to hold the fort, but once you're able to you should switch to long-term pre-emergents and more specialized post-emergents.

Should I use Scotts starter fertilizer with crabgrass preventer would that help??

That would certainly help, it has mesotrione in it as the crabgrass preventer.