I completely agree with Patrice that if it looks right - it is right.
Bases do change how people 'look' against scenery. Also, they make for wide corridors (most people do not have a 2m wide space aound them at all times) and mean that accurately-sized doors look to small (no-one would ever be able to get through a real door if they needed 1m of clearance either side).
Many real-scale buildings don't look very good on the gaming table because they 'look' too big. I think, this is because we're used to seeing things from a human point of view. From 'down there' people are the right size, and building just tower over us. from above, where we are looking down like gods, 'people' (ie minis) look like ants when the buildings are the right size. I was involved once in a conversation about building the Reichstag for 28mm minis and I think we realised the 'terrain piece' would still be about 30' or 10m long...
15mm as Patrice says does not really mean 15mm to the top of the head (it should, but it doesn't, many 15mm figs can be around 18mm). But if a figure 18mm high represents someone about 1.8m (1800mm) then that's a fairly straightforward 1:100 conversion. If it's a 15mm figure representing a 1800mm person, that's around 1:120. those scales might give you some idea as to what you can play with, but I'll repeat again, that the best guide is not some calculation of scale, but what looks right.