part-filling a pressure keg

[MostlyHarmless]

I've got 5 gallons of John Bull Traditional English Ale which is coming to the end of primary fermentation. My plan is to bottle a gallon or two for giveaways and taking to friends' houses for drinking, and to put the rest into my 5 gallon pressure bottle for home drinking off draft.

So, a quick question...when I prime the pressure keg, should I stick to roughly half a teaspoon of sugar per pint or, given that there will be more space in the partially-filled keg, should I use more sugar to get more CO2 to build up the pressure?

I'd be grateful for your thoughts on this, to help me avoid either 3 gallons of flat beer, or a potential beer bomb.

 

[prolix]

I wouldn't go for to much extra sugar but it should be fine.