AM, 50ml in 10s should be a nice flow rate, being on the slower end of the scale and I would expect extract some nice notes from the coffee with the right grind, dose and tamp. My commercial Expobar's run around 65-70 before the fitting of flow restrictors. After the jets are changed, we bring this down to around 45 - 50ml per 10 sec. A Synchro I installed with flow restrictors 3 days ago was running around 45ml in 9-10 seconds. A La Pavoni 2-group I have in my factory runs 50ml in 11 seconds. Scale buildup will depend upon the upstream protection designed in the group. Some have gauze particle filters to trap small debris, etc. It will also depend upon the material used for the jet. Older machines were made of brass that had a habit of scale sticking to the jet, whereas many newer machines use teflon.