I have a couple of observation that may not exactly please, but in the main, the Rigby forend would be quite a bit shorter (from 7 to 8 in from the front of the reciever ring) and with more taper on the underside. One little trick I use to get the proportion "about right" is to come back an inch from the end of the forend tip and have it about 1/8 in deeper at this point than the dia of the barrel. The curve up to the end (and recurve back over the barrel channel) starts from this point. This seems to keep thing in a visual balance especially if I am looking to achieve the style of the early English gunmakers.
I do hope that dosent cause offence as I am quite aware of how our pride in the work we have done on our rifles can be dented by unkind words and that is certainy not my intention. Doing any alteration to an existing stock and being pleased with it is an achievment that not many acomplish and your desire for the fine proportions of these preeminent gunmakers is a credit to your sense of style. I do aplaud the results you have achieved.