FRM 201A does state that the allowable acceptable limits, Section 6.3.5, for isokinetics is between 80-120 % or that no sampling point be outside the delta p min or delta p max during sampling.
You will recall that FRM 201A is sampling at a constant rate through a PM-10 cyclone. The objective of FRM 201A is to maintain the correct flow through the nozzle, consequently the PM-10 cyclone, to maintain the proper cut-side of the cyclone (9-11 microns) during sampling. By not sampling isokinetically, as in Method 201, we are ignoring the larger particles. Our assumption is that our bias will be small because we are only interested in the small particles (< 10 microns) which will follow the gas stream and enter our nozzle if we select a constant sampling rate that is close to 100 % isokinetics based upon the geometry of the cyclone and nozzle selection. Maintaining the proper cut-size of the cyclone is the most important aspect of this method, not the % isokinetic limit. Consequently, if I was reviewing a FRM 201A test report and they showed that the cyclone cut-size was maintained with the 9-11 micron size range, but isokinetics were 75 %, I would accept the test knowing that I am only interested in small particles (< 10 micron). Remember, under isokinetics biases the mass emission high or in favor of the regulatory agency!