Have done such data validation pairs.



On hidden sheet is a table with 1st data validation list as header. The list of data validation values for 2nd data validation list is stored in according column of hidden table. You create a Dynamic Named Range, which returns datarange from one of columns depending on value selected in 1st data validation, and use this dynamic range as list source for 2nd data validation.

NB! When you need to use this in a table (you select a pipe size in table row, and you have to select the schedule based on this selection in same row), then the Dynamic Named Range must depend from position in table too, i.e. from pipe size value selected in this particular row!

I'm really interested in what you've posted; I believe it is very much along the lines of what p45cal posted, with the exception that you've got the data table on a second or "hidden" sheet. This would be what I'd prefer to do so the front sheet can be kept visually "clean". Let me know if I'm understanding what you've proposed.