Eyeballing parallel is difficult even for the keenest observer.
Here is an easy method to ensure you start there. You'll need 2 items.
1. A very small (1") bubble level. Home Depot or your local hardware store for about $1.
2. A useless record album. I find Madonna, Donnie Osmond, or Barry Manilow work very well.
Now, drill a very small hole anyplace in the first 1/3 of the record album. Actually it can be drilled anywhere on the record but I find it more convenient with the hole close to the front. The hole must be large enough for the stylus to fit in so when the tonearm is lowered, the weight is born by the cartridge's body.
After you've lowered the tonearm into the hole and the cartridge body is supporting VTF, gently place your bubble level on the headshell. Assuming the cartridge is perpendicular to the headshell, you have a fairly accurate starting point.
As a safeguard it's a good idea to disable the anti-skating mechanism so there is no later force in play.
Good luck.