Nothing to worry about. Many moulds seem to be that way. Part of it may be that alloy the company used to determine the weight.
My Lyman #457125, the Government round nose 500 gr., comes out at 515 gr in 20:1. My PJ Creedmoor 420 gr comes out at 416 gr in 20:1. My 400 gr RCBS 40 cal comes out at 408 gr, and I've heard of others as heavy as 415 gr.
If you add more tin, you will only make the bullet a couple of grains lighter.