Freq. measurement using pin change interrupt + 4024 counter?
Your basic idea looks good to me. There are a couple points to consider.

To get a precision of 0.5% in one run of your scheme, you would need to count at least 200 cycles, so you need at least an 8-bit counter. Looks like your 4024 counter only has 7 bits.  However, that problem is easily overcome.

If you also need accuracy of 0.5%, you need to think about the accuracy of your 8 MHz crystal. (Here, precision means that your measurement is reproducible using the same setup. Accuracy means that your frequency measurement is tied to the SI second. Accuracy therefore guarantees that your measurement can be reproduced by anyone that builds a similar setup.) Basically, you are counting "ticks" of your unknown signal in terms of the "ticks" of the crystal oscillator. But what if you replace your crystal oscillator? The new one might have a different frequency. Then you would get a different answer, even if the frequency of the unknown signal didn't change.

I guess the frequency of a standard crystal oscillator is accurate to about 100 ppm = 0.1%, so you are probably OK. But check the datasheet for your crystal (again, for the accuracy spec, not precision or timing jitter). 

Finally, you might worry about the precision of your crystal oscillator - exactly how periodic is the clock signal? But over these timescales I strongly suspect it will be fine at the 0.5% level. Again, you could check the datasheet.