How to file tax after filing extension? For Federal and CA

[Kin]

Hi,

I paid 90% of my tax liability before 4/15 by filing extension and
estimated payment.

When I file my 1040 and 540,

1) Do I need to add 0.5% interest and 5% penalty (i.e. total of 5.5%)
to my unpaid tax and send to IRS and FTB? (Or Should I assume that
since I have already paid 90% of my tax liability, I don't need to pay
neither interest nor penalty?)

2) Do I need to write to state why I filed extension with my 1040 and
540 form?

Anyone has similar experience before?

Thanks in advance!
Kin