It's difficult to tell, as they have distinct methods of stimulation. Amphetamine is a Dopamine & Norepinephrine Releaser and Reuptake inhibitor, while Ritalin is mainly a Dopamine Reuptake Inhibitor. Because of this, Adderall has a distinct stimulation compared to Ritalin.
Ritalin is, however, less potent than Adderall in terms of stimulation, therefore you are in the right direction by increasing the dose. However, doubling the dose on the first try isn't the safest route to go, so I'd suggest working your way up from maybe 75 or 90mg Ritalin to see if the lower doses are enough for you, and if they aren't, you can always increase your dosage in the following trials. It's always a safe bet to start off low and be safe and perhaps waste a few dollars worth of Ritalin than to risk getting psychosis or, worst case scenario, losing your life.
IME, 25mg Adderall ER is about Equivalent to 56mg ER (IIRC) Methylphenidate, so your conversion ratio seems to be on par with my experiences. Be safe, not sorry.