Śatapatha Brāhmaṇa ("of a hundred paths") 3.5.3.19

3.5.3.19

tadāhuḥ | uttaravedeḥ pratyaṅ prakrāmettrīnvikramāṃstaddhavirdhāne sthāpayetsā havirdhānayormātreti nātra mātrāsti yatraiva svayam manasā manyeta nāhaiva satrātyantike no dūre tatsthāpayet

As to this they say, 'Let him stride three steps from the high altar westward and make the Soma-carts stop there: this is the measure for the Soma-carts.' But there is no (fixed) measure in this; wherever he himself may think fit in his mind, only not too near, nor too far (from the high altar), there let him stop them.