When looking at phase in a sine wave, for example when you are interested in wave interactions such as comb filtering, values are between 0deg and 360deg (you can normalize between 180 and -180). You are looking for the distance between peaks.
When looking at phase on a real source, such as a mono recording playing on two speakers, phase can easily exceed 360 degrees. You are looking for the distance between a specific point on two previously identical waves.
The basic math remains the same. Divide the distance difference by the wavelength of the frequency in question and multiply by 360.
So in your case: 250hz has a wavelength of 53", your difference in distance is 75", your relative phase +/-509deg, or +149deg/-211deg depending on what you are trying to discover.