So for future reference, I have figured out the solution to this problem. My main issue was that when using the max record ID option, it would change every time I entered a new record. To solve that I added additional fields.
So we have the following fields to pull in the last records value:
[Max Record ID - Helium Fill]
[Max Record ID - Date]
Now to hold that value:
[Latest Helium Fill]
[Latest Date]
Use a form rule to say if [Latest Helium Fill] is blank, fill in with the value in [Max Record ID - Helium Fill]
and one to say if [Latest Date] is blank, fill in with the value in [Max Record ID - Date].
This will fill the information into the fields, and they will hold the value.
Now I can create my calculation using the [Latest Helium Fill] and the Current one and the [Latest Date] field and the current one.
This would not work if you were creating records in grid edit.
Hopefully, that helps anyone in the future if they have a similar problem.
