I'll have to get back to you on this cause I have just finished my Kaboku sencha this morning, meaning tomorrow I will open up my 100g pack of Tsukigase Zairai.
I only brewed it once or twice myself yet I have drunk plenty before (but I wasn't the one brewing it). For what I gathered, as for good quality teas, this is rather flexible. I have even drunk this very tea "gong fu cha style" in a very small kyusu (around 10cl) using very hot water. The result was really surprising, with no flaws. It strangely made me think of a oolong (gao shan cha).
Usually, my parameters for futsumushi sencha are something like : 5-6g/10 cl, 65°C/1'30, 70°C/30', 80°C/2' and playing around from here. I'll tell you how it goes.
FYI, the article regarding this tea on Hojo's site sums up very well his thoughts about umami, as you sound surprised not to find it in your tea.