Marieb Anatomy Laboratory Manual

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I recently did a lab in class, page 55 in the Marieb Anatomy Laboratory Manual (9th edition). The lab is called Observing Diffusion of Dye Through Agar Gel. My professor, told my class to calculate the average rate of diffusion of both dyes. The thing is he expects us to come up with our own time so we can in turn plug it into the formula. I have consulted with numerous teachers and forums but no one has the right answer nor the right way to go about calculating the average rate of diffusion.

For Methylene blue I got these results: 15 minutes: 8 mm , 30 minutes: 9 mm , 45 minutes: 9 mm , 60 minutes: 9 mm

As you can see above, the dye stopped diffusing after the 30 minute mark but the lab is 1 hour long, yet the experiment calls for the person to measure the distance diffused every 15 minutes.

For Potassium Permanganate:15 minutes: 10 mm , 30 minutes: 12 mm diffused, 45 minutes: 14 mm , 60 minutes: 14 mm

I was thinking about taking the last time the dye diffused and dividing it by the final distance.

For example, for Methylene blue, 9mm/30 min=.3mm/min

And, for Permanganate, 14mm/45 min=.31 mm/min

Yet, these answers hardly make any sense because how can one make a comparison using 2 different time points

Thanks for the help and sorry if this is the wrong forum :)