Here is how you set the population of a corn planter, if you don't have a population seed chart.  First of all, make sure that on the 490 planter, the planter is set on the drill position, and not in the wire check position.  This is to make sure that the valves and the boots are open and the corn kernels don't bunch up, but instead drop straight down to the planting shoe.  There are 43,560 square feet in an acre.  Since you have a 38" row, which is 3.1666 feet, divide 43,560 by 3.1666, and you will come up with 13,756 ft.  Take 13,756 and divide it by 1000, and you will come up with 13.75 ft., which is 13 ft 9 ".  Put corn seed in one of the hoppers, preferably one of the inside ones, and drop the planter on a hard gravel driveway, where it won't penetrate the ground, and plant the corn on the driveway for about 30 ft.  Then measure where the corn is laying on the ground, 13 ft 9" and count the kernels.  If you count 25 kernels, your population is 25,000.  If you count 20 kernels, your population is 20,000.  If it does not fall in that range, either increase or decrease the speed of seed delivery by changing the shifting on the gear box or the combination of sprockets. This may take several efforts to get the planter set.  Remember this 13 ft 9" is 1/1000 of an acre.