The 80% (and the 60%) rule is BS when it comes to custom designs.
That works on the Kossel Mini, but as soon as you alter the carriages or frame, that number goes right out the window. If you use 80% on a Griffin, you will not be able to use 20% of the bed.
Also, if you plan to auto level or auto calibrate, just reaching the edge of the build plate is not enough. The further out you go, the more flat one arm will be, leaving you little to no leverage to depress the probe. Even if you do have a little extra, I recommended not probing more than 75-80% from the center of the build platform to reduce wear and tear as it is VERY hard on your diagonals doing this.
As you go out, and increase leverage, you also increase deflection, on larger printers, this can be a big problem. I usually probe at 70-80% on my smaller printer, but only 60-70% on my big printer to decrease the deflection. I use the same accuracy level, however, because as you go up in size, the same accuracy requires more and more precision. 0.03mm accuracy on a Kossel Mini may require your radius to be accurate to .01, but on a 500mm bed, to reach 0.03 accuracy across the bed may require an accuracy of 0.001 or even 0.0001. Because of this, iIt can take far longer to calibrate larger printers. Sometimes multiple runs, getting more and more precise.
I would also highly recommend that the OP use at least 20x20 rather than the usual 15x15.
That works on the Kossel Mini, but as soon as you alter the carriages or frame, that number goes right out the window. If you use 80% on a Griffin, you will not be able to use 20% of the bed.
Also, if you plan to auto level or auto calibrate, just reaching the edge of the build plate is not enough. The further out you go, the more flat one arm will be, leaving you little to no leverage to depress the probe. Even if you do have a little extra, I recommended not probing more than 75-80% from the center of the build platform to reduce wear and tear as it is VERY hard on your diagonals doing this.
As you go out, and increase leverage, you also increase deflection, on larger printers, this can be a big problem. I usually probe at 70-80% on my smaller printer, but only 60-70% on my big printer to decrease the deflection. I use the same accuracy level, however, because as you go up in size, the same accuracy requires more and more precision. 0.03mm accuracy on a Kossel Mini may require your radius to be accurate to .01, but on a 500mm bed, to reach 0.03 accuracy across the bed may require an accuracy of 0.001 or even 0.0001. Because of this, iIt can take far longer to calibrate larger printers. Sometimes multiple runs, getting more and more precise.
I would also highly recommend that the OP use at least 20x20 rather than the usual 15x15.