I have not had smoke detectors for 8 months due to Hurricane Ike.
This Site Might Help You. RE: cost to refill co2 fire extinguisher?
I'm not sure what kind of pipe you're talking about, and I'm not sure what your installation looks like. Your question is vague. If I understand you correctly, you have a straight pipe section, a 45 degree elbow, and you want to add a straight section to connect to another 45 degree elbow and pipe later. Is this correct? If so, simply cut the straight section so that you know it will be longer than the length you need. When you're ready to add the second 45, make a dry fit, mark and cut. That's the easiest and most accurate method I know of.
Do both have Multi-coating (hard + UV + anti-reflective)? If yes, then go for the 1.57.. if they cost the same, go the higher index, as the material will be slightly tougher, not to mention thinner (only slightly with your Rx). If you choose a frame which is semi-rimless (with a nylon thread), then you must choose 1.57
