How to determine phase shift through coupling gap without simulation?

How it can be done for parallel-coupled lines? For example, quarterwave-length coupling gap between two parallel half-wave microstrips gives phase shift of -276 deg. As i understand, additional -6 deg shift is because of 0.3mm coupling gap. Choosing another gap of 1mm gives -278 deg phase shift. Is there any graph or formula to approximate this value?

What is your freq?

Frequency is 10Ghz, simulation done in QUCS.