Boiler sizing dilemma

I'm trying to find a replacement boiler. I've measured the radiators at 280 ft^2. My old boiler is rated at 300 ft^2 and has a 5" flue.

Before I had the idea to replace my boiler I had my chimney lined. They sized it to the present appliances and I got a 5.5" liner.

All the boilers I can find today with 5" flues are a bit undersized, ranging from about 1% to 8% less than 280 ft^2.

The Burnham I liked is about 3% under. Presuming that I will insulate the mains, would you say I can get away with it or do I need to take steps to fit a boiler the next size up?