I have been doing industrial "stairs" for the last 46 years as a structural draftsman.
Today, Industry standard as well as a lot of building codes goes by the "7-11" rule.
Back in the day, we had another rule we went with. The height of (2) risers plus the width of the tread had to "fall" within 24" to 25". This allowed for various heights of risers and tread combinations. For instance, (2) 6.5" risers and (1) 11" tread equals 24".
I have seen risers as short as 6" and as tall as 8.5" in the industry. Once you get over that height of tread, the stair becomes a "ship ladder" type and then there are all sorts of options. Remember, the maximum height between landings for a tall stair should be 12'-0".
For our 1:48 scale layouts, I would just go with what looks good. I doubt OSHA will be visiting your layout to look for compliance! 
Hope that helps.
Donald
