Use the table for computing the tax. In your case it's $23,800 PLUS 30% of the amount over $100,000.
Don't forget to deduct your annual exclusion and I assume you did that.
If it's $150,000 gift less $18,000 exclusion then you have a $132,000 taxable gift.
So you have tax of $23,800 plus 30% of the amount over $100,000 or $32,000 times 30%.
Total = $23,800 plus $9.600 = $33,400