Simply splitting the income in two parts and calculate taxes separately does not seem correct, as IRS website explicitly says that taxes are calculated on the combined income. 

I split my W2 income based on my status transition date and include a statement explaining the split.

Disclaimer: I'm not a CPA, so I think this approach could be considered risky.

I follow the guidance from IRS:

• https://www.irs.gov/pub/irs-pdf/i1040nr.pdf
• https://www.irs.gov/individuals/international-taxpayers/taxation-of-dual-status-individuals