Rega P3 - Arm hole too small?

The arm distance pivot to spindle should be 222mm for a Rega P3.That is from the exact center of the tonearm's pivot to the exact center of the turntable spindle.Now I think you have not altered your tonearm's position from what you have said so you should be o.k.The thing that is actually more important here is how much you tighten the base nut as this has a big effect on the table's sound.Too tight and you will deaden the music too loose and the music will be diffuse.Try only tightening the nut to finger tight and then just a 16th of a turn more.Using your ears as a guide be patient as it can be labourious but try to listen for the 'sweet' spot,the sound will open out when you get it right.Yes it is worth doing.

As long as you can tighten the base-nut and the arm is the correct distance for the most part it should be o.k.Now what could also be done is to make support that would cover the hole area so you are tightening the base-nut flush and firmly to the bottom of the plinth.I cut a square piece of hardwood with a 23mm hole in it, that I can use for this purpose.The material is up to you but I would avoid MDF(too dead).Do not make it too big,mine is about 65x40mm and about 10-15mm deep.The regular base-nut can be tightened on to this.Decoupling the arm fixture is said to be an advantage on the Rega if it breaks up spurious resonances.So you can experiment here with size of the support and it's material(Acrylic?).There could be an advantage in having the arm the way it is with less contact.Hope this helps.