How do run a center_biased rect_gen Pegasus construct?

Hi, I have a very common problem:    I want to insert vias at an M1-M2 intersection.   Easy enough:   write a Pegasus deck which runs rect_gen.   The problem comes when there is enough room in the M1-M2 intersection to create a 2.5 x 2.5 array of Via1's.   Well...you can't place down 2.5 vias.  You have to round down to 2.   Which rect_gen does.  BUT the resulting 2x2 array is lowerLeft shifted, when what I need for yield reasons is centerCenter.   So it's something like:

M1 and M2 -outputlayer M1M2_overlap

size M1M2_overlap by VIA1.en -outputlayer M1M2_rectgenarea

size M1M2_rectgenarea by  [  (X * VIA1.W + (X-1) * VIA.S ) - (floor(X)*VIA.W + floor(X-1)*VIA.S)/2.0 ) -outputlayer correct_rectgenarea

rect_gen -inside_layer correct_rectgenarea

...where X = the largest # of vias that I can fit in that area, and X - floor(X) = the extra odd-man-out space leftover, because you cannot place a floating-point number of vias down.

Surely this problem has been solved before?   How is this done in Pegasus?    thanks!