Restricting a max number of a column +1 in a text box on a userform

Hi,
After days of doing this project learning a bit about VBA I have a userform working close to what I want it to…….But not quite!! I’ve attached the doc to make things easier to understand.
Point of note is Col A is a log number & Col B is the Log + ID number So the first entry of say….7 would be 7 & 7 but the 2nd entry of 7 would be t & 7B.
P.s PW = 123
P.s “ N”1 is the highest number of Col A

1. I need the “Log number” box to restrict the entry to that of +1 of the maximum number on column A (Log) or less…In other words the entry must not exceed +1 of the highest number in Col A but can be less.
2. If the log (Col A) is a duplicate number then “Job”, “Location”, “Customer” & “Rig” auto fills from the original number….Eg if Col A has a previous entry of 7, and another 7 is entered (which would be 7b in the ID Col B) then as above would auto fill.