"Slippage"(if you are referencing this as it applies to traction) I'm not sure is what manufacturers are trying to communicate as much as "lead" in terms of revolutions or "rolling circumference ratios". A larger tire will revolve less as it gets taller for the same distance but will cover more ground in it rolling circumfrence. My understanding is that if the fronts are always supposed to be "leading" when 4wd is engaged or revolving more to cover the same distance as the larger rear. A larger circumference tire in front reduces the amount of "lead" as it applies to revolutions but increases the amount of rolling circumference ratio. "Lead" in this case is being measured by me in revolutions and not rolling circumference. Basically, all this is measured in ground travel. Dividing the rolling circumference of the front tire with the RC of the rear will give a certain ratio. If this percentage is within 2% of factory ratio, should be ok with no problems due to binding.
All that needs to be done is to acquire the rolling circumference of both front and rear of the old tires and divide the front by the rear. Then find the rolling circumference of the new front tires and divide that by the rears. If these figures are within 2% of each other, he's ok.
To get "rolling circumference" multiply the diameter of the tire by pi or 3.14. This gives " free hanging circumference" . To get "rolling circumference", this number is reduced by things like tractor weight and tire pressure. By multiplying the "free hanging circumference" by 97%, one gets a fair assimilation of rolling circumference.
Hope this clears up some of the confusion from which can be a confusing topic.