Mayamatam 6.8

6.8

śaṅkudviguṇamānena tanmadhye maṇḍalaṃ likhet
pūrvāparāhṇayośchāyā yadi tanmaṇḍalāntagā

In its center one should draw a circle with a diameter twice the size of the gnomon.
If the shadow in the forenoon and afternoon reaches the circumference of that circle,