I have found 700g of mushroom powder fits pretty reliably into one one 00 gelcap. Mind you that was with a pretty uniform capping device that can be easily and cheaply purchased from a health food store. Seriously, 00 caps aren't that hard to find, hell if you're lazy pull out the yellow pages and call a few places first to check they have them in stock first. Otherwise Google it.
Below are the measurements of caps so you can work out what size your current vitamin caps are (likely 00).
000 - length 26.14mm, Width 9.55mm
00 - L 23.30mm, W 8.18mm
0- L 21.20mm, W7.34mm
IF you decide to hand fill (REALLY frustrating and inaccurate), like Cowboy Mac said, its very easy to get 150mg+/- differences, especially if the mushroom powder isn't a uniform consistency. If you still decide to go down the hand filling path, make sure you tamper the powder (push it down so as fit more in) several times for each cap (and again make sure you this uniform too - i.e. 3 tampers for every cap).
The whole point of drying and capping mushrooms is so that you can measure a reliable dose. Handfilling or 'about a teaspoon' measures kinda negate the purpose of capping it in the first place. I know I'd hate myself if I unknowingly gave someone a heroic dose, regardless of the outcome.