I researched this topic and couldn't find anything pertaining to mods having any kind of threshold but want to double check.
Correct, there is no guidance specifically related to contract modifications. But a contract modification is a "contract action".
From FAR 1.602-1(b) “No contract action shall be entered into unless… including clearances and approvals, have been met”.
These are often called "business clearance reviews". The DFARS does not provide any supplemental or implemental language, but each of the Service Components and the 4th Estate Federal Acquisition Regulation Supplements do (e.g. NMCARS 5201.690(b) requires HCA ‘s to develop written procedures and thresholds.). Typically, the major command or local guidance establishing clearance procedures take into account the contract action's dollar value, contract type, scope or complexity, or level of competition. In your case there is most likely a Navy or SYSCOM guidance that specifies your action requires a legal review or something because of the dollar value.