It's not that critical as long as you stay between 1/2-to-1 tsp, whatever that weighs. Here is what I do:

Instructions for gelatin fining of beer:
 
1) Stir 1 tsp of Knox brand unflavored gelatin in 2/3 cup of distilled water. Most grocery stores will carry Knox gelatin. The gelatin sold at homebrew shops is no better and probably identical but a lot more expensive. Knox works perfectly (but it must be *unflavored*).

2) Microwave the gelatin mixture in 15-second increments to get up to 160F, stirring between increments. Be careful not to exceed 170F or you will have Jello instead. At 160F the gelatin will be sufficiently sanitized for addition to your beer.
 
3) Pour the gelatin mixture into your carboy. 

4) Keep the carboy in the fridge until the beer is clear (approx. 2-3 days). The gelatin will not clear well if the carboy has not been kept cool.