Rounding a number with an Exponent

[repjr]

I have a formula set up that returns very large numbers that I have formatted with exponents. I'm trying to figure out how to round the number down to 1.00E+n. For example, if my formula returns 3.658E+17, I want to round it down to 1.00E+17.
Alternatively, if it can't be rounded that way, is there a way to use just the exponent from the calculated answer in a formula in an adjacent cell?

 

[steve the fish]

Like this maybe?

=10^(LEN(3.658E+17)-1)

 

[repjr]

Like this maybe?

=10^(LEN(3.658E+17)-1)

Thank you!!! That works perfectly!!