After these posts I was playing around with the calculator a bit, but can't seem to replicate the issues in regards to not being able to add pumps;
3/8" thickness with MP40's and MP10's in the calculation
1/2" thickness with MP40's only in the calculation
1/2" thickness with MP40's and MP10's in the calculation
I will pass along your suggestions, but please make sure to send your feedback on the tool to EcoTech directly, through the link at the bottom of the page aswell.