Yes, you can ride to a target Focus Duration and strain goal for the day by monitoring Xert Connect IQ data fields on your Garmin. Namely, Focus/Difficulty. TTR/TTE, MPA/Power, and XSS/XEP.
In your example, you would ride irregularly spaced intervals (terrain and traffic permitting) at your 8-minute power or a bit above. Then rest and repeat for however long it takes to accumulate 122 XSS.
This thread dives into the concept and includes a link to a YT video demonstrating the process –
As you practice riding to Focus, I think you’ll find you can closely match the target strain ratio and Focus for the day.
The advantage to XSS Buckets is that is combines the components into one data field/app and provides a visual XSS ratio bucket to fill.
Another option is to run Autogen to generate a simple workout that matches today’s goals.
View the workout details and note number of intervals, watt target, duration, and rest-in-between time then ride intervals similar to that during the ride. No need to push the workout to the Garmin but you could if you want to. Otherwise, the intervals don’t have to be exact to reach the Focus Duration and strain goal for the day. For example, Autogen might show 3 sets of 3 intervals at X watts for X seconds separated by X minutes rest-in-between. At some point during my ride, I’'ll insert 9 intervals at approximately the target watts with suggested rest-in-between.