Dear iva,
the calculation of the brush surface itself is complicated but straightforward, the cleaning validation near to impossible.
To calculate the surface you need the following data:

Diameter of the nylon bristles: In our case, it was between 0,18 and 0,25 mm. 0,18 mm is the worst case (don´t average, surface is not linearly dependant from diameter). Measure with micrometer.

Length of bristles, emerging from the central cylinder: Measure with caliber, in our case average 13 mm.

Amount of bristles per bundle of bristles: Count. In our case it was between 127 and 173, average 150.

Amount of bundles of bristles per brush: Count. In our case it was 38 rows, 19 with 45 bundles and 19 with 46 bundles.

Diameter and length of central cylinder.
Now the calculation is simple, only calculate the surface of each bristle as a cylinder, multiply by amount of bristles and bundles, sum the surface of the central cylinder. It is not necessary to substract the surface of the holes in the cylinder (where the bundles are inserted) because you can approximate it with the surface of the “tops” of the cylindrical bristles.
Now: The issue is that each brush has the surface of a whole piece of equipment (in our case each brush has a surface of 19169 cm²!), so the use of non dedicated brushes lowers your acceptance limits to undetectable levels, and when it gets to the cleaning validation, sampling it by rinsing with an appropriate solvent, you will probably find the entire product history of the brush, because to my knowledge there is no economically feasible cleaning method to remove the contamination from the brushes. My advice is obviously to get dedicated brushes, or to go for nonuniversal feeders at the blister lines.
Best regards
Alfred