I have always had a rule of thumb on sizing transformers for bids that is in the 65% range. once i exceed 65% load on my bulb wattage count, I go up to the next size. this gives me some wiggle room to add fixtures as the customer wishes, maybe bump up wattage if I need to, go to higher taps, etc. this has always worked for me and i rarely had to replace an installed trans with a larger unit unless it involved a large upgrade later. this year, tho, since letting folks know a per fixture price, I have been exceeding 80% all over the place and had to replace transformers that have been just installed as folks have added on way more fixtures than they used to. So I am starting to change my philosophy and going closer to the 50% threshold. but it still seems silly for me to install a 600 watt trans for a 180 watt load. what numbers do you guys go by ?