You can only take 50-75mL of this stuff safely. If that is what you want I mean... Not a high in any sense of the words really. I wouldn't call the feeling of 100-150mg diphen relaxing either... unless you want to sleep I mean. Keep in mind it's not at all like something such as Valium or Xanax.
If you're intent on try this, take 50mL today and save the rest for tomorrow. Get your feet wet, I mean, before you make the calm, cool, calculated rational decisions to take 75mL+ and send yourself to the hospital.
To second Tripman, diphen is not exactly a recreational drug. Do you like being delirious? Read up on Erowid's Diphenhydramine and DXM vaults before you even try to get high off this stuff, please.