I would suggest you start by actually testing for your 15RM. Once you have found it, add 20% to get your 10RM. Then add 15% to your 10RM to estimate your 5RM. So if your 15RM for bench is 200lbs, you can estimate that your 10RM will be around 240lbs. And further, if your 10RM is 240 your 5RM will be around 276lbs. When estimating your maxes, be conservative. It’s better to guess too light than too heavy.