Running wire to a shed

I work in a home improvement store doing administrative work and was asked about a wiring project today, I only have very minimal knowledge of electrical and understand not to make suggestions in this field when unsure so I had to hand the customer over to another associate but I'd like to know more about how to complete the desired project.

The customer wants to run a 30 Amp circuit from his house out to a shed in his yard. In the shed he will just have a few lights inside, a security light outside and two or three outlets inside. he said he would mostly just have a small compressor and some light duty air tools in use in there. He was going to run 10 AWG underground feed wire out to the shed in some buried conduit. The first part that got me stuck was the complication of voltage drop, I have red you shouldn't get into a situation where there is more than a 5% drop, he is running the wire about 100 - 120 feet and by using an online calculator to estimate voltage drop it gave me about a 4.99% drop at 100 feet. I was wondering what the actual equation is to estimate a drop is and if he would have to bump up to 8 AWG or can run 10/3 versus 10/2 to have the extra hot leg carry some more power.

My second question was that he wanted to have a switch in his house to control security lights mounted to the outside of his shed. I wasn't sure about the way to wire that for such a long run.

Thank you for any insight into this project and apologies if any of this is obvious and I am just unaware!