First response time

In Call Log API reference there is "startTime" field in record and "startTime" fields in legs:

https://developers.ringcentral.com/api-reference/Call-Log/readUserCallLog

Please help me understand how can I calculate the "first response time", i.e. how long the caller waited until the callee answered.

Can I take some leg and calculate difference between it's "startTime" value and record's "startTime" value?

Or, maybe, I should use some other field for this purpose?