Connect up forms and tabs to create pricing on a Google Sheet

I have a google sheet that I would like to generate a price based on different information on different tabs. I have uploaded an excel version of the sheet but the code needs to work in Google Sheets as this is the format of the live version.

There are sizes (column AJ and AK) to work out a square meter size. The minimum chargeable size is 0.3sqm so anything under this size needs to be charged at 0.3 minimum. Column H contains a product code of either "DGU" or "Single".

If DGU is selected then the input information is in columns AL:AQ using the corresponding prices on tab "sealed unit pricing".

If "Single" is selected then the input information is in Columns AR:AT using the corresponding prices on tab "Single sheet pricing".

Finally the line item total calculated from the above would be multiplied by the quantity in column J with the final price input into the column K.