Just a few quick tips on protecting your spreadsheet.
Passwords & Cell Protection
There are limits to the security provided by passwords.
Worksheet password protection can be removed by software you can find on the internet.
Workbook password protection is stronger, but can still be removed, especially if you use short or common passwords.
To protect formulas remove cell protection from input cells and protect the sheet with a password.
Longer passwords, with mixed content are best.
Use symbols or numbers instead of letters
@ for a
1 for I
8 for B
0 for O
5 for S
+ for T
2 for Z
! for 1