I can recommend the SBOOI - the Leitz 50mm external finder - it's nice and bright, and 1:1 so you can use it with both eyes open all day long.
If it makes you feel any better, I've found the Rollei 35 frame lines not all that accurate. At least not for the original version (no idea about later models.) I have a Retina Ia and the viewfinder leaves much to be desired. Fortunately it has an accessory shoe, so if you have a 50mm finder you can just slip it in and use that instead.I have a Rollei 35 and a Kodak Retina but I can't focus accurately enough with the former and the later isn't that ergonomic.