Is there a formula/ratio for calculating how much Lantus someone would take based on how much Humalog they use for basal needs? It doesn’t seem to be a 1:1 ratio.

We are planning a one day pump vacation while at the waterpark. The endo can tell us how much Lantus to use, but I would like to be able to figure it out myself for future events. There are formulas based on weight in some books I have, but my son takes considerably less insulin than his weight would indicate, so those formulas don’t work for him.

Thanks.