You can preserve the angle with a level and a stack of quarters. Stack as many quarters as needed on the low side to get the level to sit level. Don't spend them until you've leveled the new saddle.
That said, whatever measurements you make, don't think of them as rocket science. Saddles vary, they sag differently, and you sit on them in different places, so make reasonable effort to match the position, but consider it only as a starting point. Then fine tune based on feel.
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