Notes Payable (Imputed Interest Rate Estimated, Stated Interest Rate, Amortize Difference)

Accounting for the imputed (estimated) interest rate on a Note, Approximate an Applicable Interest Rate on Note that differs from Stated Interest Rate, and where the market interest rate is not determinable and the note is not marketable , must impute the interest rate, example is for On 1/1/20X1 Corp-A issued a pomissory note to Design Corp for design costs on a building, the Note has a face value of $500,000, due 12/31/20X3 with a stated interest rate (4%) payable at the end of each year, Corp-A cannot determine the fair value of the design services, nor is the Note marketable, therefore on the basis of Corp-A's credit rating, no collateral involved, prime interest rate at that date, and prevaling interest on Corp-A's other debt, etc., Corp-A imputes a (10%) interest rate on this Note issued, what interest rate should be used for recording this Note (4% stated or 10% imputed) ?, What interest rate should be used given the choice ?, Based on this imputed (10%) IR the fair value of the design services costs (present value of Note issued on 1/1/20X1) can be determined and the interest expense on the Note is calculated using both the stated (4%) rate and imputed (10%) rate using the (Effective Interest Method)