Whether to start harvesting or not depends on how much you want. If it's a couple of leaves go ahead. I use mint in quantity and a single harvest would denude those plants entirely. You can pick mint whenever you need it through the growing season but it will die to the roots come winter. If you consider dried mint worth having make sure you get some organised before the plants die down. Btw it doesn't always flower. In good, rich moist conditions it might not even bother. And if does start blooming just cut the flowers back and it will keep producing plenty of tender leaves.

Some suggest that cutting it to one inch above ground is best like you had mentioned. But would it come back again would like to harvest throuout the growing season ,so maybe a few leaves at a time would be the best way ?

These plants are still tiny. So I wouldn't cut them back severely. Established mints can take a hard haircut. But it isn't an exact science. There isn't a 'best' way. Mint is tough. You can cut it wherever and whenever you like once it's attained a bit of size and it will regrow.

Ok I am fine to let it grow I have just read that Mint should be harvested just before flowering which starts happening in June. Maybe I would just start to cut 1/3 at a time??

You are vastly overthinking this and you seem a bit scared of your plants. Mint is virtually indestructible. You can harvest it any time you like. It just tastes a bit better and is more tender if it's not flowering. For tea that doesn't matter at all but if you're going to eat the leaves it might. It may not even flower at all and it certainly doesn't work to a calendar. Cut what you need when you need it. As long as you keep it fed and watered it'll grow back.