This is a 2 in one bed calibration for the Tevo Little Monster (350mm Bed). IF you have made some of the recommended upgrades for the Tevo Little Monster, in this case the manual bed level, then this little print will not only let help get you bed manually level and perfectly square, it will also allow you to fine tune your delta radius to compensate for any bed dishing.
By manually squaring the printers carriages to the top of plate of the printer with no x/y/z ensdstop off sets, then using the nozzle to square the bed up to the top plate ensures good first layer adhesion but more importantly it ensures print accuracy. The use of automated bed level on a Delta leaves the dimensional accuracy of the prints pretty poor, fine for an ornament but horrible for functional parts. You just use the auto bed level to adjust for different bed surface Z-heights if you have also upgraded to removal glass bed (only do a type 4 level).
Run a pretty hot nozzle and bed, I suggest 75C bed and 215C nozzle. This ensures a real nice smooth and fast extrusion so you can print at 100mm/s .
The idea is to print the outer ring a couple of times adjusting the bed manually on the 6 points till you get the level perfect all the way round. Then once you are happy and the outer ring has printed with no adjustments let it start the next ring, you are looking to see if the nozzle is too high or too low. If too high (most likely scenario) then you want to reduce the size of your delta radius. Remember when adjusting the bed level via the 6 points, adjusting 1 point will have an effect on it's 2 neighbouring adjustment points as well as the one directly diagonal across the build plate.
When you adjust the delta radius in your system settings, you will have to get the z-height level right on the outer ring again, this will generally mean you will need to adjust the it in your settings, on the Duet wifi you can just baby step away/towards the plate and note the variance and add this to the system settings once you go to change the delta radius again saving you time.
So once you have the middle ring and out ring printing right it is time to move onto the centre circle and repeat the process again. You may wish to omit the middle ring from the printing when you start making delta radius changes (so print outer ring and inner circle only) till you have your delta radius pretty close
You will be surprise how much of a difference taking the hour or so to do this calibration right (instead of using auto level) when it comes to starting a print, overall print quality and especially dimensional accuracy.
Best of luck