PackDaddy

I'm wondering if there is a way for a calculation to be done after an entry is made by a user on the front end of the site. Example: What is the weight of these 3 objects on Mars or the moon I have 3 values for weight: 10lbs, 20lbs, 30lbs I have 3 calculations for a planet/moon: Earth (just uses the original weights), moon (Weights*16.5%), Mars (Weight*38%) I'd like to have a person use a drop down menu to choose either earth (default) moon, or mars and the weights would change on the front end based on the calculation I also can't just make all 9 calculations on my sheet because on my actual spreadsheet there will be hundreds of entries that I don't want to replicate, and also, both the values and calculations can change. Thank you