What he said, topeak d2 is a good guage.
Sensible answer.. The thumb test is only good enough to let you know whether your tyres so flat your in danger of getting snake bite punctures or not (that's when the rim hits the ground and pierces the inner tube).
The correct pressure for you depends on a lot of factors, your weight, your riding style, the type of tyre you have.
Too much pressure and the ride will be harsh, too little and you'll get snake bites.
A digital guage is a good idea so you can tune the pressure to your personal preference.
For example, 25psi is too low on my mountain bike, much above 30psi is too firm. 27psi is just about right.
There's no way you can tell a 2 or 3 PSI difference just by gripping the tyre with your hand.
Edit, road bikes will generally run a lot more pressure than that, but the key thing is to get a reliable pressure guage, so you can repeat readings, compare like for like pressures.
Even if the guage itself is not 100% scientifically accurate, you'll have a repeatable benchmark to check against, and cheap alalog gauges can be all over the shop.