You're correct. Since phase differences in wave interference problems are often expressed in terms of multiples of $\lambda$ or as dimensionless values, I should present the answer in that form.

The phase difference in this case is approximately 2.54 (dimensionless). This value represents the phase shift without any units, as it indicates the relative phase position within a cycle.

Thank you for pointing that out!

Would you like additional clarification on this?

Here are some related questions for further exploration:

1. How do we interpret a phase difference of exactly 2 or 3?
2. What phase difference corresponds to constructive interference?
3. Can phase differences be negative, and what would that signify?
4. How does the separation $d$ influence the phase difference pattern on the screen?
5. How would a change in the screen position $x_P$ affect the phase difference?

Tip: Phase differences can also be expressed in terms of the fraction of a wavelength, which helps in visualizing constructive and destructive interference.