My opinion on the customer's lawn, phos is not deficient until below 20. Additional phos will not have any positive effect on turfgrass in this soil. Phosphorus is outlawed in some states due to the detrimental effects if it gets into surface water of lakes and sreams. If you were growing flowers, vegetables or corn--then yes--the phos is needed.
A regular feeding of nitrogen--with about 50 percent of the nitrogen in slow release form is all you really need, (along with water and regular mowing).