Retirement tax questions

I hope Dmertz is right but need some confirmation. As this will help me put money in my SEP correctly. I will not put anything to my SEP if it's not going to impact on my deduction. Also what's the last date on putting funds to my SEP. I have already opened one account before Oct 1 deadline but have not put any money. Is there any website where I can pretend filing taxes and see it myself if it's allowing me or not do double sure.