Without getting to far in to it, is there are hard and fast rule that if your AGI is less that x you should contribute to a Roth, if its more that x contribute to tIRA...?

No, no hard and fast rule that considers only AGI. There are rules of thumb, but there are also *many* exceptions to those "rules." The true answer depends on your marginal tax rates both now and when withdrawing.

See Traditional versus Roth - Bogleheads for more.

I looked over that page and had to run some numbers myself.

I don't like his 30% tax, but I'll use it, maybe a try percentages later.

I used $10k, 20yrs growth @ 7%, 30% tax rate.

tIRA

$10k for 20yrs @ 7% = $38,696 minus 30% tax = $27,087

Roth IRA

$10k - $3k = $7k for 20yrs @ 7% = $27,087 Oddly the same!

Non tax advantaged

$10k - $3k = $7k for 20yrs @ 7% = $27,087 - cost of $7k = $20,087

$20,087 - 30% taxes = $14,061 + cost of $7k =$21,061

If you need context it's in the article.

(taxed before investment, or taxed at withdawal, or only gain taxed)

The remaining variable is the tax rate applied at investment and withdrawal time.

I plan on starting Roth Conversions in 2018, I expect to be at the 12% tax rate,

but when my wife and I have SS, RMDs and dividends, I expect our rate to be more than 12%

I ran some more cases and I'm surprised at the results but maybe I shouldn't be.

I'm using what I hope will be my numbers for 2018, ie. 12% tax bracket with a $35,000 Roth conversion.

$35k @12%tax leaves $30,800.

$30,800 @7% for 7 years = $49,458 Roth Tax free dollars.

$35,000 @7% for 7 years = $56,202 taxable. If unconverted.

$56,202 @12% tax leaves $49,458 after taxes.

Exactly the same with or without conversion.

$56,202 @20% tax leaves $44,964 after taxes.

Let's try a 10% growth rate.

$35k @12% tax leaves $30,800.

$30,800 @10% for 7 years = $60,020 Roth Tax free dollars.

$35,000 @10% for 7 years = $68,205 taxable. If unconverted.

$68205 @12% tax leaves $60,020 after taxes.

Exactly the same with or without conversion.

$68205 @20% tax leaves $54,564 after taxes.

Growth rate makes no difference, Let's try the time frame, I'll drop from 7 years to 3 years.

$35k @12%tax leaves $30,800.

$30,800 @7% for 3 years = $37,731 Roth Tax free dollars.

$35,000 @7% for 3 years = $42,877 taxable. If unconverted.

$42,877 @12% tax leaves $37,732 after taxes.

The same with or without conversion.

$42,877 @20% tax leaves $34,302 after taxes.

And again at 10%

$35k @12% tax leaves $30,800.

$30,800 @10% for 3 years = $40,995 Roth Tax free dollars.

$35,000 @10% for 3 years = $46,585 taxable. If unconverted.

$46,585 @12% tax leaves $40,994 after taxes.

Exactly the same with or without conversion.

$46,585 @20% tax leaves $37,266 after taxes.

Summary: If you have the same tax bracket at withrawal as when doing the Roth Conversion, the end result is equal dollars, If you have a higher tax bracket at withdrawal, the Roth is a better.

Since I expect to be in a higher tax bracket at withdrawal, I will do a Roth conversion.

I'll lose some deductions as the kids finish college.

The next question, is it worth contributing to my SEP/IRA and doing a Roth conversion?

Advantage, gets more money into tax defered accounts. The SEP/IRA deduction allows more to be put into the Roth Conversion.

Disadvantage, you will have increased RMDs because of the contribution.

You can continue Roth conversions after 70-1/2.

If you have any disagreement, Please post them, I'm here to learn.