In a flow step use the change revenue stage and assign it to the MQL stage.
You should have 2 separate programs.
- Lead Scoring Program: This program should contain all the scoring that is being done when a lead is created. This would include both behavior and demographic scoring. Create a campaign in this program to push leads where the score goes >25 to 'Assign to MQL program.
- Assign as MQL: Create a campaign in this program which will be requested. In the flow, you can mark the lead as MQL, and sync to SFDC.
Now, talking about your high-value campaigns, like someone filling Contact Us form, you should create a campaign in Contact Us Program and request Assign as MQL campaign. It is very critical that you just have a single campaign which assigns leads as MQL and pushes them to SFDC.
Reach out directly if you want to see an example of a nice lead flow created in Marketo.