The $230.80 is the total premium, not the surcharge. From the linked source, the base rate is $164.90 so the surcharge is $230.80 - $164.90 = $65.90.
On the other hand, you wouldn't want to use total income in the denominator because that's an effective rate and you'd want to use a marginal rate. The
Worth pushing through the Social Security hump and/or IRMAA cliffs? wiki section has some good illustrations.
The marginal rate for going $1 over the first IRMAA tier is ridiculously large, as happens with any tax "cliff". At best (and coincidental to the quoted calculation), if one can completely fill the $52K-wide tier, the marginal rate due to IRMAA alone is $65.90 * 12 * 2 / $52K = 3%, as the second chart in the above wiki section also shows.