I dont know why you would bother with fertiliser at this time of year, yes maybe some potassium, but nothing as high as that with nitrogen. May as well piss your $55.00 up at the pub would be better. Potassium is for strength in the roots and stems, nitrogen is for leaf growth. Fertilising might extend your mowing for a bit longer but at what cost as most clients would not pay the extra to have their lawns fertilised at this time of year. If you used the high potassium fertiliser and explained why you want to do it, they may go for it.
As a greenkeeper we used to use slow release and the last dose was usually end of February early March. Fertiliser NPK was 10:5:23. Then nothing until we where getting ready to bring the greens out of winter. We would then give a high nitrogen fert of say 32 in late August or early September just to get the grass growing, dependant on frost zones, high frost zone we would leave it until late September or early October. We would then do a renovation in early October and we would feed the greens high nitrogen and potassium with lots of other minerals. Topsoil and water then every 8 to 12 weeks we would fertilise again right through to the end of Summer.