CalcItNow is completely revamped. It's now the only ReST API native integrated formula suite for Marketo (also works stand-alone). Check it out on Launchpoint: CALCITNOW - Field formula calculations – Marketo LaunchPoint
Try using Webhooks to create your formula: Using Formulas in Marketo .
I haven't tried it yet so can't vouch for it, but if it works, please let us know.
You also can ask Marketo Service to enable you and set up the Munctions feature that can sum up 2 score fields in Marketo.
Support is not able to set up this feature. Marketo Professional Services used to be able to set up this feature, but they no longer offer it.