Regarding loss of Shakha we can make calculation based on the no. of present Shakha. For instance Kurma Purana Purva Bhaga Chapter 52 gives this data;
एकविंशतिभेदेन ऋग्वेदं कृतवान् पुरा ।
शाखानान्तु शतेनैव यजुर्वेदमथाकरोत् ।।
सामवेदं सहस्रेण शाखानां प्रविभेद स ।
अथर्वाणमथोवेदं विभेद कुशकेतन ।।
भेदैर्ष्टादशैर्व्यासः पुराण कृतवान् प्रभुः ।
सोऽयमेकश्चतुष्पादो वेदः पूर्वं सनातन ।। (Kurma Purana 1.52)
Initially, the Rigveda was divided into twentyone branches, while the Yajurveda was divided into a hundred branches. Thereafter, Vyasa who dwelt in the house of Kus'a, distributed Samaveda into a thousand branches. Atharvaveda too was also distributed into nine branches. Besides this, Vyasa composed eighteen types of the Puranas. Thus in the ancient times there had been a single Veda, which was subsequently divided into four parts.
If we add the total Shakhas of Veda we come to know that originally there were about 1130 Shakhas. Now, as I discuss in my answer here only about 12 - 13 Shakhas of Vedas are surviving. If we calculate in percentage the loss of Shakha is about 98.94%. ie. It means about 98% Shakhas of Vedas are lost upto now.
Now, regarding loss of verses (Richas/Suktas), Samhitas of same Vedas do not differ so much wildly in content. For instance in my answer here I have presented variation of Sri Rudram Hymn in various Shakhas. And regarding content we can't be sure how much percentage of Vedas we have lost.