Hello,

This is decided on the basis of what is needed in the form.
Mostly, marks of the major 5 subjects are needed to be entered in any form you fill.
If your board gives you percentage basis all the subjects including optional and that percentage score is higher than the aggregate percentage of major 5 subjects, go with the former.

Ultimately you must follow these simple rules:
1.) Go with the higher percentage score of either major 5 subjects or all subjects (inclusive of optional too) : if nothing is specified.
2.) If specified in the rule book, go accordingly. Some may suggest you to enter aggregate of best 5 (in that case use the top 5 scores to calculate percentage) while some may strictly ask you to ignore optional subject.

Hope this helps!!
For any other do comment
All the best !!