Tempdb should be some % size of Large Database in SQL Instance ?

It doesn't seem like that!!!

There is NO any arithmetic formula to calculate Tempdb Size. Yes ?

Do we have any ?

Yes. still we can calculate/figure out by performing the following things when storing Intermediate results

1. Memory Spill - Causing by Hash Or Sort operation
2. Rebuilding Index along with SORT_IN_TEMPDB Option
3. DBCC CHECKDB on Larg Database
4. Using Temp (#/##) Table
5. Using multiple aggregations with huge data

Tempdb doesn't behave like User Databases (Say suppose, If the User database grows up to 500 GB and It'll remain same even the SQL Instance restarts. Right ?

But, The Tempdb will be recreated with the Size to whatever the size it was last set to!!!

To avoid Memory Spill:
- Omit the ORDER BY clause if you do not need the result set to be ordered.
- If ORDER BY is required, eliminate the column that participates in the multiple range scans from the ORDER BY clause.
- Using an index hint, force the optimizer to use a different access path on the table in question.
- Rewrite the query to produce a different query execution plan.
- Force serial execution of the query by adding the MAXDOP = 1 option to the end of the query or index operation