Computer Power Supply to bench top theory
The math works and there's a good question / answer Choosing power supply, how to get the voltage and current ratings? that should help reassure you on that side of things. Powering a small circuit from a power supply is not an issue with one exception I can think of:

The power supply will still be capable of delivering a large amount of current in short circuit and other unexpected conditions that present as a low resistance. I have a 40A bench supply and recently while testing something used some low-current test leads and fried those in an instant when I accidentally shorted them. Some circuit failures can also be more dramatic / dangerous when a large amount of current is available.

For that reason for use as a general bench supply I'd recommend adding a fuse, it depends on what you're doing but perhaps a 500mA - 1 A fuse would be a good choice because that should blow before test leads and component leads that short have a chance to melt.