First of all try some debugger-enabled build of dosbox, on mode changes
you'll see additional information like this:
VGA:H total 56, V Total 260
VGA:H D End 40, V D End 200
VGA:Width 320, Height 200, fps 61.461952
VGA:double width, double height […]
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VGA:H total 56, V Total 260
VGA:H D End 40, V D End 200
VGA:Width 320, Height 200, fps 61.461952
VGA:double width, double height aspect 1.200000
The fps calculation (which seems to be used later for the avi as well)
is in vga_draw.cpp:
LOG(LOG_VGA,LOG_NORMAL)("H total %d, V Total %d",htotal,vtotal);
LOG(LOG_VGA,LOG_NORMAL)("H D End %d, V D End %d",hdispend,vdispend);
fps=(float)clock/(vtotal*htotal);
No idea if the clock value is correct, or what else causes the difference
from the values reported, but you can try to force an arbitrary fps
value there.
Also you might want to have a look at hardware.cpp (avi writing).