I tend to class those earning above £50k/year as fertile ground for the Conservatives. However I would add the £30k retired who own property and shares etc. The comfortably off retired are big voters. Another test are those that send their children to fee paying schools or pay for tutor support for their chidren who are in state schools.
You can imagine my surprise when the Redcar seat went Conservative. The seat includes South Bank, Normanby, Eston, Grangetown, Dormanstown. To me very working class areas with a lot of Labour councillors. Not many 40% income tax payers.
East Cleveland and SE Middlesbrough is a bit different - Saltburn for example has always been a bit of a middle class area to me - Guisborough has a lot of lower middle class, with alot of home ownership, Nunthorpe and Marton similar, if anything Nunthorpe is almost middle class with a lot of professional workers. Its a mixed constituency.
In 2019 a lot of Teesside voters wanted the area to be rejuvenated and the Conservative message seemed to be believed on that score more than Labour's. It may be different in 2023/4, but Labour are suffering a bit from not doing enough on the rejuvenation score when they have had opportunities to do so. Brexit obviously hindered the Labour vote in 2019 and Corbyn's image as a 70 year old in a Che Guevera T shirt did not helo his cause in a small conservative area of the country for politics. The image was unfair in my opinion but it existed.