I would suggest the following:

For higher frequency use a fixed prescaler e.g. 1/256.

My frequency meter work with the AT89C2051 programmed in C.

T0 work in mode 3 with TL0 as counter for the divided input frequency and TH0 as time base. Both are expanded by its overflow interrupts to 24 bits.
For low frequency range the input frequency was feed direct into T1 as 16 bit counter.

Then the frequency value was given as:

val = count / time * XTAL / 12;
if( lf_mode == 0 )
val *= PRESCALER;


So my frequency meter can measure from 0.5Hz ... 125MHz with 5 digits or 0.01Hz resolution

Peter