"Scale effect" is a divisive subject but I'll remain objective in my response.
507C was a Light Grey giving a very obvious and distinct light impression. In any scale an accurate 507C will wholly fail to give an impression of Home Fleet Grey.
As to 507B - it wasn't "medium grey". Primary source documentation traces its existence back to the First World War where it DID start as a discreet colour from 507A but it survived WW1 and 507A did not. 507B would evolve through the 1920s and 30s gaining blue pigment and dark grey enamel to make it hardier and longer lasting. Enamel was expensive and made the paint somewhat glossy, and in 1939 the nomenclature "507A" was resurrected with identical description and ingredients as 507B except the enamel was omitted. 507B was effectively discontinued in 1940 when enamel use was ordered to be suspended.
The point of the above is that 507B did exist but it wasn't what you think it was. Unfortunately this mistake has been repeated and reprinted ad nauseum in many books so it will take a long time to kill it off and replace the general understanding with the true one.
The impression of Home Fleet Grey wad dark. If you're used to Snyder & Short's chips or those model paint brands which copied them then you are correct to shy away from using the 507A - it's too dark at circa 8% Light Reflectance Value versus a documented 10-13%. This is enough to make it look wrong on a model. The "507B" measures out at 20% which is much, much too light to give an impression of Home Fleet Grey in the common scales. If building in 1/2400 or something miniscule lile that it might look ok.
There are some selected relevant things here you might find interesting. In particular our little PDF paper on the Admiralty Pattern 507s will provide more information on the above and shows objective comparisons between the Snyder & Short chips and primary source documentation uncovered since. Having instrumentation that can be hand carried is a step forward in technology since too which eliminates the human eye margin for error.
https://www.sovereignhobbies.co.uk/pages/british-royal-navy-colour-schemes