For UHF, it might be better to convert to BNC fittings for minimum loss. Note the symmetry of the layout in the photo. This distance is not critical, but use care to place the whips in a perfect square pattern on the roof and align the array sides parallel to the sides of the vehicle. It's tedious to set out the mag-mount whips using just a ruler or tape measure. Cutouts in the template corners match the whip bases.
Emplacement is fast and misalignment is avoided with the templates.
Once the whips are in place, I remove the template so it doesn't get rained on or blow away. When changing bands, check the bearing accuracy of your setup using a known signal source before setting out to find an unknown emitter. In my case, readjustment of the calibration control has not been necessary when whip length is within 20 per cent of resonance. On the other hand, using two meter whips on 224 MHz gives about 90 degrees bearing error. The presence of other VHF antennas on your vehicle may cause bearing error on some bands.