I've added a quick version using the round one in the note below.
AND I've modified that one too :)
With just the arcs and not a continuous rim, there are sometimes bumps at the ends that mess up the measurements so here is one without the gaps. The picture shows the earlier one with gaps.
As I can easily take the build plate off for measurements, there was no need for it to hold together. This quick one makes multiple readings for adjustments much faster and uses less filament.
Also, there is a 100mm vertical test piece that needs to be run first to adjust the "DEFAULT_AXIS_STEPS_PER_UNIT"

NOTE!!!!
I recommend you don't use my original, but look at this one that was based on it...
http://www.thingiverse.com/thing:1557870
A much better job as it is actually smooth around!

Test print for adjusting a Delta Printer that has uneven prints.
Make sure it is printed centrally on your printer.

The notes here are from torsti76 on the Deltaprintr forums, thank you :)

"Fortunately, Marlin has a (currently undocumented) cure for that (cf. https://github.com/MarlinFirmware/Marlin/issues/2159).
In short, what you should do is:
print a cylindric object of a relatively large diameter, say 100 mm
measure the dimensions in the direction of each tower
if you find they are off, adjust DELTA_DIAGONAL_ROD_TRIM_TOWER_x, where x is the number of the tower (1 - front left, 2 - front right, 3 - back)
The formula for adjustment would be:
(DIAGONAL_ROD_LENGTH intended dimension for tower x / measured dimension for tower x) - DIAGONAL_ROD_LENGTH = DELTA_DIAGONAL_ROD_TRIM_TOWER_x
Assuming you measured the dimension for the X tower (front left) to be 102 mm for your 100 mm cylinder, the formula for adjustment would be:
(255 100 / 102) - 255 = -5
So, you'd put that line in your Configuration.h:
"#define DELTA_DIAGONAL_ROD_TRIM_TOWER_1 -5"
This way you'll be able to eliminate all dimensional failures without having to painstakingly measure your printer or to modify its basic mechanical settings."

This thing was made with Tinkercad. Edit it online https://www.tinkercad.com/things/9rnbCDI4tTc

EDIT:
Original had no aligning marks so it was deleted and the modded version replaced it.
Now there are pictured of where to measure it.

Deltaprintr

Deltaprintr Kit

No

No

.3mm layers

20%

Test to see if the Delta Printer is symmetrical.

Following from discussions ..
https://github.com/MarlinFirmware/Marlin/issues/2159
and
http://forums.deltaprintr.com/index.php?threads/most-recent-marlin-version-adapted-for-the-deltaprintr.732/page-3