You have no idea what a 250μ milled finish is, until you see it and feel it. These gauges are relatively cheap, durable, and a quick reference. Having one around will make your drawings more accurate and ultimately save machining time on your parts.
Below is a handy reference chart from the Wikipeida article on surface finish.
Here are some gauges you can order from Amazon. The less expensive one is cast and not calibrated. This means that it’s not standards compliant, and not as accurate as the calibrated scales. In my experience, this calibration is generally unnecessary with surface finish gauges. Learn about surface finish calibration at the National Institute of Standards & Technology’s website.