Well, your larger ship is around 14.500 tons in size. This means, you need maintenance facilities on earth for ships of that size (73 if my math is right) to keep their maintenance clock from advancin.
What you have to keep in mind is this:
As the clock advances, the chance for maintenance failers increases. This means, your 50% chance becomes 100% chance after a while, then 200%, then 400 % (this takes quite some years, mind you) and after a long time, things will get so much out of control, that the ships start to explode.
You can think of this as your ships becoming 30 years old rust-buckets that simply start to fall apart.
To counter this, put the ships into overhaul every two or three years - for which you also need enough maintenance facilities.
As a rule of thumb, I calculate my fleet needs like this:
1/3 in overhaul
1/3 on the way to/from mission area
1/3 on a mission
Usually, this is a bit on the cautious side, leaving me with more ships ready than I realy need, but it´s better to err on the side of caution.
