Split-grade printing is based on an approximation -- an approximation that is poor in some common situations. Here's a paragraph
@Nicholas Lindan wrote in an app-note for split-grade:
Traditional split grade printing is based on the naive assumption that the black and white points can be controlled independently with the high and low contrast filters. This, however, is only true for prints made at close to normal contrast with roughly equal amounts of high and low contrast filtration - a grade 2 print - which beggars the point of why bother with split grade at all if it only works at one grade. Some pundits favor always making the high contrast test print first and some the other way round - followed by lot of hand waving and assurances that experience will allow the printer to modify the result of the test prints as needed - but this again negates split grade printing’s promise that it will find exactly the right contrast grade for a negative and that experience is not required.
The key to making split grade printing work reliably is realizing that high and low contrast prints
must be treated differently.
Here's the link to this app-note:
Split-Grade Printing: A Measured Approach.
This app-note describes a method of split-grade printing using his easel-meter (it's for sale, and I own one) and
no test-strips. The method requires taking two meter-readings, so I suspect you could create two test-strips in lieu of the two readings. Then you should obtain the desired highlights and shadows more reliably.
I wrote "should" because I have not tried this method because I programmed the equations into my LED-controller based on meter-readings.
Mark