Kegs & bottles - at the same time

A few weeks back I filled several bottles from my keg of summer ale (just a simple ale with some rye in it). I don't have Blickmann gun so I just used a piece of tubing pressed into my picnic tapper nozzle to fill the bottles. I topped the bottles off so that there was foam coming out the top and then I capped them. I waited about 3 weeks to try them and found that they really didn't have as much carbonation as the beer that comes out of the keg.

I imagine that over time I could perfect the process since I have read that many people do this. However, I have two different brews in the fermenter and I'll be giving some of this out to friends for Christmas. I am thinking that when it comes time to keg them, I will rack off 2 gallons from each batch into my bottling bucket and rack the rest into my kegs. That way, I know the bottled beers will be as well carbonated as the kegs (and as a bonus, I can sample each and compare).

So my question is this - other than calculating the required amount of sugar to use in the bottling bucket for a 2 gallon batch, is there anything else I need to take into consideration. I still have about 2 weeks before the beers will be ready to transfer so that gives me plenty of time to think things through.

Yeah I use 1oz of sugar per gallon with good results. If you slightly over carbed the keg when you bottle from the keg the beer would retain more co2 in suspension to give your bottle more pressure. Most people bottle from the keg just to avoid bottle sediment. So yes all you need to worry about is the proper amount if sugar and the appropriate amount if time for the bottle to carb up.

You'll retain a little more carbonation if you freeze your bottles as cold as they'll get before filling with the improvised beer gun. Using a bottle filler helps, too.