I'm blasphemous to some in the sense that I do not have a thermometer, the truth is I left it at school though I seldom use it there either.

So My question is when preparing sencha "how does one correctly gauge the correct temperature water?"

I have the water cooling in my cup and Ususally once its barely steaming I pour it in. This seems to be working very well, and I have enjoyed the teas I've produced that way, but I feel it might be a little on the cool side as I remember sencha-s usually packing more of a punch, especially Fukamushi's.

So is there a formula such as rougly 30 seconds in a room temp cup after boiling? Or should I watch the steam rising from it and touch the sides of the cup like I have been doing?

Actually, I prefer to go without a thermometer! I get the feeling of being more intimate with the tea, and less complicated when going without the thermometer. Still, to each their own, and some love using it.

Anyway, I think it's best to use all your senses. Go by the amount of steam, by touch, by the amount of time since you've poured the water and however else you get a handle on the water's temp. Also (as you would appear to have already done) start off by using a thermometer so you have something to accurately gauge your intuition to.

For me the cups measure the water amount and the teaware regulates the temperature, pour boiling hot water in kyusu and from the kyusu tranfer into the cups, and there you go you now have the right temperature, put leaves immediately in your kyusu and pour water from the cups back into the kyusu wait and pour back into the cups, it really doesn`t need measuring, with 200 ml water the temperature using banko kyusu drops to 72 - 75 C with 300 ml water and my tokoname kyusu it drops to 78 - 80 C, I only measure temperature with gyokuro, with sencha and chinese greens I meaured until I learned and now I do not need a thermometer, I find myself using it rearely, when I bought it, I felt it was indispensable, but now it sits in my drawer.

Different strokes for different folks.

I use a thermometer for Japanese greens virtually every time. Whatever works for you.

Brew how you like, like how you brew!

I'm a very touchy-feely person when it comes to tea. I don't really pay too much attention to getting the parameters exactly right. I just smell, look and feel to see if it's ready, and that gets me perfect results.

Tea is what you want it to be, and I just want it to be relaxing. I'd go mad if I had to measure everything.

Which is why I always say, brew how you like, like how you brew!

I know from experience that by just following my normal pattern of water cooling, that for the first steep I am w/i around 5* F of where I want to be, heh, but I like finding the sweet spot and hitting it every time if I can. I am not relazxed if I don't ... weird as that may sound.

2nd steep I pay a little attention to temp, going mostly intuition. 3rd steep on, no measuring of anything.

Have you seen the movie Matrix? There is a scene where Neo wakes up and said "I know Kung Fu", it was one day when I woke up and said "I know temperature without measuring".

I used a thermometer to figure out how long to microwave my water...now I just put the water in for 3 minutes 20 seconds and it is perfect every time

I cheat. I have a digital kettle that I can set to the temp. I need.