I use Seavision as well, and mine is bifocal.The problem I have with the various guage reader masks I have tried is they use a thick glue on magnifier. This allows some water to sit on the top edge of the glued on glass, and that makes for an imperfect sightline.
I finally found SeaVision masks that have the guage readers molded in as a concave form rather than an convex, external step.
They now are making glue on gauge reader and Rx lenses too.
They do it all, and have several nice mask models in stock.
Chad