This might sound a little dumb but here it goes!
I have a high-end PC, so everything I code and test runs absolutely smooth! The problem is, I can’t figure out how it will look on a regular PC.
How do you guys manage this?
Have a test machine, or possibly a vm with small cpu/ram caps.
Also intuition.
I have a medium end computer for development, if I get hundreds of fps on that one then I hope it will work on low end computers too 
I use dynaTrace to profile the cpu/memory usage to try to get rid of potential bottlenecks on low end computers.
Mike
I will go ahead and assume, given you have a high end PC that you can probably shuffle aside £300-400 for a testing rig.
Personally I would not care, just develop the game and worry about performance when you start getting a community and churning out performance patches.
everyone has an old laptop lying around or knows someone who does =P
If only there was somewhere on the internet with a big group of people , like a forum , where members are willing to test out the software on different computers? would be great!
By the time you release the game, your hardware will be considered regular 