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The gap is usually 3mm all round (apart from the bottom which depends on floor surface etc) so a gap of 3.5mm should be fine!
si_mon628-
The old school way of hanging a door is to use a pound coin as a gauge, so for the final offering up you aim for the door to be held in by a pound coin on the hinge side, lock side and head with a wedge holding it up to the head and no lateral movement.
Pros judge each other by how even the gap is round the door once it's been shot in (assuming the gap isn't too big or small). The only time building regs get involved is with fire doors in which case the gap must be more than 2mm and less than 5mm (an even 3mm gap being gold standard) this is allow the intumescent strip and cold smoke seal (if fitted) to function properly in the event of a fire.
Bear in mind that any timber door (even shitty hollow core jobs) will be subject to seasonal movement so the initial gap will change.
Bobbo
If a door frame is 820mm wide can I get away with an 813mm door?
First time doing internal doors and this width is the only one that goes to 2m high.