I was looking for something like this, but couldn't find anything exactly like it. So I learned to code and made my own. There might be a better way to do it, but this works for me.
This spreadsheet shows the exact percentage you are towards your dragon's desired level.

Step 1: make a copy of this spreadsheet: [TEMPLATE] Namira's Progress to Level Calculator.
Step 2: Put your dragon's name in the column labeled "Name."
Step 3: Put your dragon's desired level in the column labeled "Level goal."
Step 4: Put your dragon's current level in the column labeled "Current level."
Step 5: Go into the coliseum and find the first number (before the slash, see image below) on your dragon's experience bar (either in "Organize Party" or after a battle). Put that number in the column labeled "Progress bar #." This will give you an exact view of your progress including the experience you've earned since reaching your current level, rather than relying on only the experience at the start of your current level.

Do not edit the red fields in columns E-G or the level reference in columns J-K the auto calculation will break.
That's it! Just keep your current level and progress bar number updated as you gain experience and the sheet will automatically calculate the percentage you are towards your goal. Once you get to 100%, the progress percentage box for that dragon should turn green if I did it right.
Please let me know if you find any errors or have any questions! I'm running on coffee and not enough sleep right now lol.
This spreadsheet shows the exact percentage you are towards your dragon's desired level.

Step 1: make a copy of this spreadsheet: [TEMPLATE] Namira's Progress to Level Calculator.
Step 2: Put your dragon's name in the column labeled "Name."
Step 3: Put your dragon's desired level in the column labeled "Level goal."
Step 4: Put your dragon's current level in the column labeled "Current level."
Step 5: Go into the coliseum and find the first number (before the slash, see image below) on your dragon's experience bar (either in "Organize Party" or after a battle). Put that number in the column labeled "Progress bar #." This will give you an exact view of your progress including the experience you've earned since reaching your current level, rather than relying on only the experience at the start of your current level.

Do not edit the red fields in columns E-G or the level reference in columns J-K the auto calculation will break.
That's it! Just keep your current level and progress bar number updated as you gain experience and the sheet will automatically calculate the percentage you are towards your goal. Once you get to 100%, the progress percentage box for that dragon should turn green if I did it right.
Please let me know if you find any errors or have any questions! I'm running on coffee and not enough sleep right now lol.