The 10 ounces (x) of your starter will have the same hydration as your 27 ounces of starter, that is, 145%. By definition, the amount of water (w) in your x ounces of starter will be 1.45 times the amount of flour (f) in your x ounces of starter. That is, w = 1.45f. Doing a bit of simple math, it turns out that the amount of flour (f) is equal to x divided by 2.45. That is, f = x/2.45. So, if x equals 10 ounces, the amount of flour is 10/2.45 oz., or 4.08 ounces. The amount of water is 1.45 times that amount, or 5.92 ounces (you can alternatively subtract the 4.08 ounces from 10 ounces). If you divide the 5.92 ounces of water by the 4.08 ounces of flour, you will see that the ratio comes to 145%.
Now, to add more flour to the 10 ounces of starter to get the total to a hydration of 65%, you would use the expression w/(f + f1) = 0.65, where f1 is the additional flour you will have to add to the 10 ounces of starter to get to a hydration of 65%. The amount of flour f1 needed can be expressed as
f1 = (w-0.65 f)/0.65
So, plugging in w = 5.92 ounces and f = 4.08 ounces in the above expression, solving for f1 will give you a value of 5.03 ounces of flour that you will have to add to your 10 ounces of starter to get to a final hydration of 65%. If you divide 5.92 ounces of water by the total flour (4.08 plus 5.03, or 9.11 ounces) you will get 65%.
So, to use the above expression for f1, you will first have to solve for w and f based on the quantity x and the hydration percent of your starter.