I use an1800 mah Life also. The way I look at it, go big with what you can fit in the box. You typically add some lead to the radio box anyway so that weight might as well be usable power. Truth is, now that I switched to Life packs I probably only chew through 130mah per race day. Even with the nut cracker of a steering servo I have in there.
I run a 500mah pack in my 21 mono and only use about 65mah after a day with that too.
Brian