In terms of getting a neat and consistent parting line on split moulds, it is nearly always going to be impossible to get popular flange making materials right up to the pattern unless it's a completely straight/flat piece or, in the case of curved shapes, you have the means to machine cut the flange material to exactly match the patterns profile. Even then, there's always a chance of some gel coat leaching under the flange material as it's brushed into the angle, I think

On the last split mould that I made, I used narrow strips of 5mm sheet wax to create the parting line and then cut out the flange material to fit as closely as possible to the surface of the wax strip. Filleting wax was then used to fill the slight gap between wax strip and the polypropylene flange, which was supported by foam blocks temporarily glued to the reverse and stepped slightly to overcome the width of the sheet wax. 

It resulted in a very slight step in the split above the sheet wax, but the consistent angle line it created and the fact that it is adhesive backed made things a bit more straightforward

Just an idea, should anyone be struggling to get a neat parting line, especially on curved items where getting filleting wax neatly applied might be tricky