"choose a birth year (+-2), then add 4 years and another 4 years etc" You can squeeze another year out - when you choose a year and plus / minus 2, that's actually 5y, so you can jump up in 5s. To keep track, I simply start with the multiple of 5 below and work up in 5s - e.g. start 1840+/-2, then 1845+/-2, etc.... That's easy enough to remember for me. Adrian ​