Be very careful if you decide to change the route of administration as it multiplies the effect considerably, oral = 10%, others are much higher, IV = 10x that of oral.
The Extended-release tablets come in: 5 mg, 7.5 mg, 10 mg, 15 mg, 20 mg, 30 mg, and 40 mg. With your tolerance I think 20mg Extended Release and below would be fairly safe, but long lasting.
Now if they're Instant Release than a 30% drop wouldn't be a bad idea but with the tolerance you describe, 20mg would also be fairly safe and not last near as long. Exercise caution in taking large doses of an ER med as you don't know how you'll react, but be under the influence for quite awhile vs Immediate Release. It should take about 20-40 mins to feel so don't redose any sooner than 90 mins and titrate up slowly as it can turn on you.
Opana IR doses
Use of OPANA as the first Opioid Analgesic. Initiate treatment with OPANA in a dosing range of
10 to 20 mg every 4 to 6 hours as needed for pain. Do not initiate treatment with doses higher than
20 mg because of the potential serious adverse reactions .
https://www.drugs.com/dosage/opana.html
Opana ER Conversion
Rough estimate of conversion
[TD="colspan: 2"]CONVERSION FACTORS TO OPANA ER[/TD]
[TD="class: Toprule"] Prior Oral Opioid[/TD]
[TD="class: Lrule Toprule"] Approximate Oral
Conversion Factor[/TD]
[TD="class: Toprule"] Oxymorphone[/TD]
[TD="class: Lrule Toprule"] 1[/TD]
[TD="class: Toprule"] Hydrocodone[/TD]
[TD="class: Lrule Toprule"] 0.5[/TD]
[TD="class: Toprule"] Oxycodone[/TD]
[TD="class: Lrule Toprule"] 0.5[/TD]
[TD="class: Toprule"] Methadone[/TD]
[TD="class: Lrule Toprule"] 0.5[/TD]
[TD="class: Toprule"] Morphine[/TD]
[TD="class: Lrule Toprule"] 0.333[/TD]
Stay Safe! You can always add but not subtract what you've taken.