Thank you Jacob for the advice. I will try first to maintain the original angle if it is reasonable for the type of saw. Fortunately, I do not own saws with missing teeth.I forgot to add - the simplest way to sharpen a saw (if it was OK before it needed it) is to pick up the angles already used by laying you file into the gullet until it's a close fit, then carry on filing at that angle. Work alternate gullets one side, turn and do the other side. You get a feel for it - best if you work from end to end without stopping so that you don't change the angle, nor file the same gullet twice. A missing tooth or two - just carry on as if they were still there, pressing down into the gullet. After a few sharpenings the tooth will have reappeared.
Don't set it until you have tried it - it might not be needed.
Much easier than trying to impose your own chosen angle or other radical saw doctoring