I found the best way to tell if the ballast loss is by the temperature of the ballast. It ought to feel very slightly warm( 5'C above room) as if warmed by a 5W bulb or less.
Does it?
Is it the kind that all tubes fail if one fails?
Then consider a modern ballast which is more efficient and has independent outputs. I used Quad 32W 4' tube ballasts which I can use for any number from 1 to 4 tubes. I choose tubes that compete with LEDs around 89 Lumens per Watt , tri-phosphor true daylight 5000'K with 85% CRI values. Tubes usually only come in bulk of 20 tubes and last 33k to 50k hrs.
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