When typing formulas on the Forum for people to read, I would suggest using the {code} style which indents and switches to a monospaced font so it's easy to see significant spaces. You need square brackets "[style name]" for a style to be recognized.
If you want all strings to be a specific length, then appending that number of spaces and taking the leftmost characters is probably the most efficient way of doing this:
LEFT({string expression}&"as many spaces as you need",n)
where "n" is the required length of the string.
This will evaluate slightly faster than calculating the number of spaces needed each time.
I think Sam missed Bruce's comment that the his formula (which uses "MID" rather than "LEFT" so needs two parameters) required 25 spaces to be specified. The reformatting of the reply for the Forum removes the duplicate spaces unless the {code} format is used.
Bruce's formula would display as:
MID(TRIM([Item number fieldname])&" ", 1, 26)