For Employee SIMPLE IRA contributions, the IRS info does not mention contributions required to be a % of salary; just a total for the year ($12,500 for 2018, plus $3,000 catch-up if eligible), and that deposits must be made within 30 days after each month.
For Self-Employed persons, deposits must be made by January 30th for the prior year.
More info:
https://www.irs.gov/retirement-plans/simple-ira-plan-faqs-contributions
If your business issues you a Corrected W-2 and you applied for an Extension, you should still be able to report your contributions for 2018.
Click this IRS link with info on Correcting Plan Errors.
[Edited 3/12/2020 | 6:31 PM]
**Say "Thanks" by clicking the thumb icon in a post
**Mark the post that answers your question by clicking on "Mark as Best Answer"
