I am looking for a very simple formula to calculate the ratio of water to 99% isopropyl alcohol that I need to prepare a solution with a density of 0.925 g/cm3. I am assuming there is one.
If performing an interpolation still passes as «simple» for you, you may find experimentally determined values about iPrOH in water tabulated for example in journals about (chemical) engineering. The previews of this and this give you about 10 entries for free (which may be complemented by the entries in the CRC Handbook of Chemistry and Physics like here). If more data are required, a more recent here suggests going to a library and having a look into Ullmann's Encyclopedia, too.
While keeping an eye on the temperature of the two, determine the volume of water and iPrOH to mix in separate containers in advance (1L of water and 1L of iPrOH don't add up to 2L of blend).